The first evidence of multicellular animals appears in fossils from the late Proterozoic era, about 575 million years ago, after the last snowball glaciation. These impressions were made by soft-bodied organisms such as worms, jellyfish, sea pens, and polyps similar to modern sea anemones (Fig. 17). In contrast to the microorganisms that dominated the Proterozoic era, many of these fossils are at least several centimeters long, and some measure up to a meter across.

Shortly after this time, starting about 540 million years ago, something extraordinary happened: the incredible diversification of complex life known as the Cambrian Explosion. Within 50 million years every major animal phylum known in fossil records quickly appeared. The Cambrian Explosion can be thought of as multicellular animals' "big bang"—an incredible radiation of complexity.

What triggered the Cambrian Explosion? Scientists have pointed to many factors. For example, the development of predation probably spurred the evolution of shells and armor, while the growing complexity of ecological relationships created distinct roles for many sizes and types of organisms. Rising atmospheric and oceanic oxygen levels promoted the development of larger animals, which need more oxygen than small ones in order to move blood throughout their bodies. And some scientists believe that a mass extinction at the end of the Proterozoic era created a favorable environment for new life forms to evolve and spread.

Following the Cambrian Explosion, life diversified in several large jumps that took place over three eras: Paleozoic, Mesozoic, and Cenozoic (referring back to Fig. 4, the Cambrian period was the first slice of the Paleozoic era). Together these eras make up the Phanerozoic eon, a name derived from the Greek for "visible life." The Phanerozoic, which runs from 540 million years ago to the present, has also been a tumultuous phase in the evolution of life on Earth, with mass extinctions at the boundaries between each of its three geologic eras. Figure 18 shows the scale of historic mass extinctions as reflected in marine fossil records.

Early in the Paleozoic most of Earth's fauna lived in the sea. Many Cambrian organisms developed hard body parts like shells and bones, so fossil records became much more abundant and diverse. The Burgess Shale, rock beds in British Columbia made famous in paleontologist Stephen Jay Gould's book Wonderful Life, are ancient reef beds in the Canadian Rockies of British Columbia that are filled with fossil deposits from the mid-Cambrian period (footnote 9).

Land plants emerged between about 500 and 400 million years ago. Once established, they stabilized soil against erosion and accelerated the weathering of rock by releasing chemicals from their roots. Since faster weathering pulls increased amounts of carbon out of the atmosphere, plants reduced the greenhouse effect and cooled Earth's surface so dramatically that they are thought to have helped cause several ice ages and mass extinctions during the late Devonian period, about 375 million years ago. By creating shade, they also provided habitat for the first amphibians to move from water to land.

The most severe of all mass extinctions took place at the end of the Paleozoic era at the Permian/Triassic boundary, wiping out an estimated 80 to 85 percent of all living species. Scientists still do not understand what caused this crisis. Geologic records indicate that deep seas became anoxic, which suggest that something interfered with normal ocean mixing, and that Earth's climate suddenly became much warmer and drier. Possible causes for these developments include massive volcanic eruptions or a melting of methane hydrate deposits (huge reservoirs of solidified methane), both of which could have sharply increased the greenhouse effect.

The Mesozoic era, spanning the Triassic, Jurassic, and Cretaceous periods, was the era of reptiles, which colonized land and air more thoroughly than the amphibians that preceded them out of the water. Dinosaurs evolved in the Triassic, about 215 million years ago, and became the largest and most dominant animals on Earth for the next 150 million years. This period also saw the emergence of modern land plants, including the first angiosperms (flowering plants); small mammals; and the first birds, which evolved from dinosaurs. Figure 19 shows a model of a fossilized Archaeopteryx, a transitional species from the Jurassic period with both avian and dinosaur features.

Another mass extinction at the end of the Mesozoic, 65 million years ago, killed all of the dinosaurs except for birds, along with many other animals. For many years scientists thought that climate change caused this extinction, but in 1980 physicist Louis Alvarez, his son, Walter, a geologist, and other colleagues published a theory that a huge meteorite had hit Earth, causing impacts like shock waves, severe atmospheric disturbances, and a global cloud of dust that would have drastically cooled the planet. Their most important evidence was widespread deposits of iridium—a metal that is extremely rare in Earth's crust but that falls to Earth in meteorites—in sediments from the so-called K-T (Cretaceous-Tertiary) boundary layer.

Further evidence discovered since 1980 supports the meteor theory, which is now widely accepted. A crater has been identified at Chicxulub, in Mexico's Yucatan peninsula, that could have been caused by a meteorite big enough to supply the excess iridium, and grains of shocked quartz from the Chicxulub region have been found in sediments thousands of kilometers from the site that date to the K-T boundary era.